code: def text_to_instance(self, source: str, target: str = None) -> Instance:  # type: ignore
      fields = {}
      fields["source"] = TextField(self.tokenizer.tokenize(source), self.token_indexers)  # type: ignore
      if target is not None:
          fields["target"] = TextField(self.tokenizer.tokenize(target), self.token_indexers)  # type: ignore
      return Instance(fields)
  
imports: from typing import List, Iterable, Dict
  import torch
  import pytest
  from allennlp.common.testing import requires_gpu
  from allennlp.data.instance import Instance
  from allennlp.data.dataset_readers import DatasetReader
  from allennlp.data.data_loaders import MultiProcessDataLoader, WorkerError
  from allennlp.data.fields import Field, TextField, MetadataField, TensorField
  from allennlp.data.tokenizers import PretrainedTransformerTokenizer
  from allennlp.data.token_indexers import PretrainedTransformerIndexer
  from allennlp.data.vocabulary import Vocabulary
  from allennlp.data.data_loaders.data_collator import LanguageModelingDataCollator
label_desc: Use image node_id 3 for calling the MockOldDatasetReader obj's underlying member method code with example usage: obj.text_to_instance(source, target) and returns: Instance